1. Field of the Invention
The present invention relates to a dielectric resonator apparatus for use in the microwave and millimeter-wave frequency bands, such as a dielectric filter, a voltage-controlled oscillator, or a dielectric duplexer, and to a high-frequency module using such a dielectric resonator apparatus.
2. Description of the Related Art
Large-capacity, high-speed communication systems are required in response to the rapid increase in demands for a mobile communication system and the use of multimedia in the system. As the amount of information to be communicated has been expanded, the frequency band in use has been extended from the microwave frequency band to the millimeter-wave frequency band. In the millimeter-wave frequency band, a TE.sub.01.delta. -mode dielectric resonator, which has been known conventionally and is formed of a column-shaped dielectric, can be used in the same way as in the microwave frequency band. Since the frequency of a general TE.sub.01.delta. -mode dielectric resonator is determined by the exterior dimensions of its column-shaped dielectric, fine machining precision is required. When a dielectric filter is made by disposing a plurality of TE.sub.01.delta. -mode dielectric resonators at a specified interval in a metallic case, since the coupling between a dielectric resonator and input and output means such as a metal loop, or the coupling between a dielectric resonator and another dielectric resonator is determined by the distance therebetween, highly precise arrangement is required.
The applicant of the present invention has proposed a dielectric resonator and a dielectric filter having fine machining precision for solving these problems, in Japanese Patent Application No. 7-62625.
FIG. 5 is an exploded perspective view of a dielectric filter according to the foregoing application.
As shown in FIG. 5, a dielectric filter 101 includes a dielectric substrate 102 and electrically conductive plates 103a and 103b.
The dielectric substrate 102 has a certain relative dielectric constant. It is provided on both its main surfaces with conductors 102a and 102b both having two circular openings such that the openings are opposed to each other.
On the surface (the upper surface in FIG. 5) close to the conductor 102a of the dielectric substrate 102, an input coplanar line 104a and an output coplanar line 104b are formed such that they are disposed close to the two openings.
The electrically conductive plates 103a and 103b are disposed and secured such that they sandwich the dielectric substrate 102 with gaps provided from the dielectric substrate 102 near the openings. The input coplanar line 104a and the output coplanar line 104b protrude from the electrically conductive plates 103a and 103b. The electrically conductive plate 103a is provided with grooves such that it is not connected to the input coplanar line 104a and the output coplanar line 104b. The electrically conductive plate 103a is electrically connected to the conductor 102a of the dielectric substrate 102, and the electrically conductive plate 103b is electrically connected to the conductor 102b of the dielectric substrate 102.
With this structure, electromagnetic energy is trapped in the dielectric substrate 102 near the portions sandwiched by the openings of the opposing conductors 102a and 102b to form two resonant zones. Each resonant zone serves as an independent resonator, and the adjacent resonators are coupled to form a filter having two resonator stage.
As described above, since the resonant zones are specified by the size of the openings of the conductors, etching and other methods can be used in manufacturing, and a dielectric resonator can be created in which required precision in the dimensions of a resonator in relation to a frequency is satisfied with extreme accuracy. The input and output means and a dielectric resonator, or a dielectric resonator and another dielectric resonator can be disposed with a high positional precision to obtain the desired coupling strength.
In the dielectric filter 101, since the resonators formed of the dielectric substrate 102 sandwiched by the openings of the opposing conductors 102a and 102b have a high capability of trapping electromagnetic energy, when an input and output means is formed of the coplanar lines 104a and 104b, the coupling between the resonators and the input and output means is weak and that coupling is strengthened by making the distance from the openings of the conductors 102a and 102b to the coplanar lines 104a and 104b as narrow as possible.
A voltage-controlled oscillator has also been known conventionally as an apparatus using a dielectric resonator. FIG. 6 shows a voltage-controlled oscillator.
As shown in FIG. 6, a voltage-controlled oscillator 111 uses a column-shaped TE.sub.01.delta. -mode dielectric resonator 112.
The TE.sub.01.delta. -mode dielectric resonator 112 is disposed on a printed circuit board 113 through a support base 112a having a low dielectric constant. On the lower surface of the printed circuit board 113, a ground electrode (not shown) is formed. The printed circuit board 113 is covered by an upper metal case 130 and a lower metal case 131.
On the printed circuit board 113, a microstripline 114 serving as a main line and a microstripline 115 serving as a sub line are formed such that the TE.sub.01.delta. -mode dielectric resonator 112 overlaps with them in the vertical direction in the figure.
The microstripline 114 is connected to a ground electrode 117 through a chip resistor 116 at one end, and is connected to the gate of a field effect transistor 118 at the other end.
The microstripline 115 is connected to the ground electrode 117 through a varactor diode 119 at one end, and the other end serves as an open end.
The field effect transistor 118 is connected to an input electrode 122 through a microstripline 121 at its drain, and is connected to one end of a microstripline 123 at its source.
The input electrode 122 is connected to a chip capacitor 120 in parallel to the microstripline 121.
The microstripline 121 is connected to a matching stub 124 at the connection point with the drain of the field effect transistor 118.
The microstripline 123 is connected to the ground electrode 117 through a chip resistor 125 at the other end. The microstripline 123 is formed at the middle in parallel to a microstripline 126 at a certain distance such that they are electromagnetically coupled.
The microstripline 126 is connected to an output electrode 128 through a chip resistor 127.
The output electrode 128 is connected to a chip capacitor 129 in parallel to the chip resistor 127.
With the above configuration, the varactor diode 119 changes its capacitance according to an applied voltage, and hence the resonant frequency of the TE.sub.01.delta. -mode dielectric resonator 112 is changed and the oscillation frequency changes.
As described above, in the dielectric filter 101 shown in FIG. 5, since the coupling between the coplanar lines 104a and 104b and the resonators formed of the openings of the conductors 102a and 102b is weak, the coupling between the resonators and the input and output means is strengthened by making the distance from the openings of the conductors 102a and 102b to the coplanar lines 104a and 104b as narrow as possible. If the openings of the conductors 102a and 102b join exposed portions of the dielectric substrate close to the coplanar lines 104a and 104b, the electromagnetic fields of the resonators are disarranged and the filter characteristics may change. Therefore, the distance between the openings of the conductors 102a and 102b and the coplanar lines 104a and 104b is limited.
The input and output coplanar lines 104a and 104b are formed on one main surface of the substrate 102. Since the length of the substrate extends in the direction in which the resonators are arranged, the dielectric filter 101 also becomes long. Therefore, the space for input and output means such as the coplanar lines 104a and 104b prevents the dielectric filter 101 from being made compact.
In the voltage-controlled oscillator 111 shown in FIG. 6, the strength of the coupling between the TE.sub.01.delta. -mode dielectric resonator 112 and the microstripline 114 serving as the main line and that between the resonator and the microstripline 115 serving as a sub line are determined by the relative distance between the TE.sub.01.delta. -mode dielectric resonator 112 and the microstripline 114 and that between the resonator and the microstripline 115. Therefore, the TE.sub.01.delta. -mode dielectric resonator 112, the microstripline 114, and the microstripline 115 need to be disposed with a high precision.
Since the electromagnetic field of the TE.sub.01.delta. -mode dielectric resonator 112 expands widely around the TE.sub.01.delta. -mode dielectric resonator 112, it may couple with microstriplines other than the microstriplines 114 and 115, such as the microstriplines 121 and 123. This unnecessary coupling may make the oscillation frequency of the voltage-controlled oscillator 111 unstable. Conventionally, to suppress problems caused by such coupling, patterns are designed such that the microstriplines 121 and 123, which are not to be coupled with the TE.sub.01.delta. -mode dielectric resonator 112, are disposed as far as possible from the TE.sub.01.delta. -mode dielectric resonator 112.
Placing the microstriplines as far as possible from the TE.sub.01.delta. -mode dielectric resonator 112, however, means that the printed circuit board 113 becomes large, and as a result, the voltage-controlled oscillator 111 itself also becomes large.
In addition, since patterns are designed under the condition that the microstriplines 121 and 123, which are not to be coupled with the TE.sub.01.delta. -mode dielectric resonator 112, are disposed as far as possible from the TE.sub.01.delta. -mode dielectric resonator 112, the pattern design has a low degree of freedom.
The TE.sub.01.delta. -mode dielectric resonator 112 is disposed on the printed circuit board 113, and the printed circuit board 113 is covered by the upper metallic case 130 so that the electromagnetic field of the TE.sub.01.delta. -mode dielectric resonator 112 is trapped. Since the height of the upper metallic case 130 needs to be made larger than that of the TE.sub.01.delta. -mode dielectric resonator 112, the height of the voltage-controlled oscillator also becomes large.